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CRUSHING AND GRINDING                              197
which the rock is broken. Due to the small gyrating movement, each nip accomplishes only a small part of the total work, a number of successive nips being required to break the rock from its original size to the finer product of the machine.
Gyratory breakers are much more efficient than those of the jaw type with regard to tonnage produced, whether on a basis of horsepower consumed or total weight of the machine. The accompanying table shows the comparison of Blake jaw type and gyratory breakers based on equal width of opening in each case. It will be seen that the superiority of the gyratory over the jaw breaker increases with the size of the machine up to a certain limit.
There has been considerable argument over the comparative advantages of the jaw and gyratory type of rock breakers; one of the principal advantages claimed by the advocates of the former being that less head room is required for its use. This apparent advantage has been found to be fallacious in general. By superimposing the drawings of jaw and gyratory breakers of equal opening width, it will be found that there is little if any difference between the two as regards the difference of elevation between the point of intake and the point of rock discharge.
There are, however, cases in which it is desirable to use breakers of the jaw type. In general, these are where a limited capacity of rock of certain size is required. For example, should it be required to produce 200 tons per 24 hr. of rock of maximum initial size of 15 in., it will be seen that the jaw breaker has a capacity of about 48 tons per hour, whereas the gyratory has a capacity of about 100 tons per hour. Since the gyratory machine is much heavier, it will be more expensive to install. Consequently, the jaw-type machine, although having a great excess over the required capacity, would be advisable because its capacity is not so enormously excessive as in the case of the gyratory, and, is a lighter and cheaper machine to install. In many cases where rocks of large size are to be handled, even this advantage disappears, since it may often be much more economical to operate a large gyratory breaking machine for a small fraction of the total time than continuously to expend power, which is excessive per unit, by means of the jaw-type machine. Where continuous operation at maximum capacity is expected, or where uniformity of product is desirable, the gyratory machine is far preferable.
In both jaw and gyratory types of rock breakers, mechanical efficiency should be considered, as with any machine for any purpose. Rock breakers are not all alike and some of them have advantages which ought to be carefully considered in the selection of machines for any particular work.
There has been a great deal of discussion and difference of opinion with regard to the proper selection of a rock breaker for primary breaking. Rock coming from a mine or quarry is made up of pieces of various sizes ranging from fine granular pieces up to 5 or 6 ft. in diameter. In general, it may be said that the average piece will not be more than 24 to 36 in. in diameter. Some authorities advocate the installation of a primary breaker capable of handling the largest piece, while others advocate a breaker only large enough to handle the average pieces, breaking up extremely large ones with dynamite before taking them to the machine. The advantage of having a breaker of large size is that it avoids the expense of dynamiting or sledging with its consequent large employment of labor. Its disadvantage is that when a breaker is large enough to take the pieces of extreme size, there is a great excess of capacity, requiring the machine to be idle a greater part of the time. The large machine will, in addition, represent a greater investment than the smaller one and will require more power while it is actually in operation. In cases, which are by no means unusual, where a large breaker may be operated at definite periods when other equipment is not requiring power, thus avoiding a high power peak, it may be ultimately economical to use a large breaker, doing away with the expense of dynamiting and